Light emitting device contact layers having substantially equal spreading resistance and method of manufacture

ABSTRACT

A light emitting device and method of fabricating a light emitting device. The device contains a light emitting junction, such as a pn junction. Contacts ( 11, 15 ) to the junction are designed to have substantially the same spreading resistance to produce a substantially uniform voltage across the light emitting junction. This will produce substantially light emission by the junction. The contacts can include contact layers ( 11, 15 ) whose spreading resistances are substantially matched by controlling the doping, thickness and/or composition of the layers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 60/046,437, filed May 14, 1997.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to light emitting devices and,particularly, to LEDs and a method of manufacturing LEDs designed tohaving a uniform potential across the area of the light-emittingjunction.

2. Discussion of the Background

There are a number of methods for the fabrication of discrete LEDs.Although the fabrication of LED arrays are constrained by issues relatedto the fabrication of discrete LEDs, there are additional constraintswhich must be considered when optimizing the fabrication of LED arrays.

For example, discrete LED arrays are typically fabricated on aconducting substrate since isolation between devices is not a concernbecause the devices will be separated as part of the packaging process.Conversely, LED arrays may require isolation between elements which iscomplicated by the use of a conducting substrate compared to the use ofa semi-insulating substrate. Although a conducting substrate can beused, the fabrication is complicated and typically requires additionalisolation and/or planarization to ensure that proper electrode isolationis achieved.

In addition, discrete LED fabrication typically uses a top contact nearthe center of the LED and a bottom contact around part or all of theperiphery of the device. This approach minimizes issues associated withspreading resistance of the bottom contact due to the thickness of theconducting substrate but spreading resistance associated with the topcontact can result in a non-uniform voltage at the pn junction resultingin LED brightness nonuniformity.

Since the pn junction is forward biased during operation, and the pnjunction has an exponential current characteristic in the forward biasmode, a small difference in junction voltage significantly changes theoperating current. According, the light emitted by the pn junction isnonuniform. Typically, this is manifested by a greater brightness nearerthe top contact.

Solutions to the spreading resistance problem of the top contact haveincluded the use of transparent contacts and additional process steps tominimize the spreading resistance between the top ohmic contact to thedevice and the pn junction. Thick epitaxial layers and/orsemi-transparent contacts have been used. These solutions are not idealsince 1) transparent contacts typically exhibit some spreadingresistance and these transparent contacts are not as transparent as nocontact at all and 2) the growth of thick epilayers adds additional costand complexity to the process and results in a thick device. Forexample, currently a high brightness LED is being manufactured having avery thick layer of GaAsP on top of the LED, but the device (epitaxiallayers) is about 10 μm thick.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an LED and a methodof manufacturing an LED where the spreading resistance in the upper andlower layers of the LED structure is matched to obtain a uniform voltageacross the pn junction of the LED structure.

Another object of the present invention is to minimize the spreadingresistance in the upper and lower layers of a LED structure to simplifythe matching of spreading resistance and achievement of a uniformvoltage across the pn junction of the LED structure.

Yet another object of the present invention is to allow light emittingdevices to be fabricated on a semi-insulating substrate to simply thefabrication process.

A further object of the present invention is to reduce the absorption(bandgap and free carrier) in the top layer of an LED structure whilematching the spreading resistance in the upper and lower layers of theLED structure.

A still further object of the present invention is to minimize thethickness of an LED structure while matching the spreading resistance ofthe upper and lower layers of a LED structure to simplify planarizationin the fabrication of LEDs and LED arrays.

These and other objects of the present invention are achieved by a lightemitting device where contacts are made to the light emitting junction.The spreading resistances of the contacts are substantially equal,resulting in a substantially uniform voltage across the light emittingjunction and substantially uniform light emission by the junction. Thespreading resistance can be substantially matched by controlling thecomposition, doping and/or layer thicknesses of the contact layers.

The junction can be a pn junction having a p-type active layer and ann-type clad layer. The contact to the active layer may include aclad/grad layer or layers and a p contact layer, while the contact tothe n clad layer may include an n-type grade layer and an n-type contactlayer.

The layers through which light is emitted should be made transparent.This may be accomplished by making these layers thin, controlling theircomposition, or selectively removing portions of these layers in thelight emitting area. When the layers are selectively removed, thespreading resistance matching can be accomplished with the remaininglayers and/or layer portions.

The above and other objects of the invention may also be achieved with amethod of fabricating light emitting devices where contacts to a lightemitting element are formed have substantially the same spreadingresistance. By forming the spreading resistances of the contacts to besubstantially equal, a substantially uniform voltage across the lightemitting element is produced, resulting in substantially uniform lightemission by the element. The spreading resistance can be substantiallymatched by controlling the composition, doping and/or layer thicknessesof the contact layers.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a diagram of an LED according to a first embodiment of thepresent invention;

FIG. 2 is a diagram of an LED according to a second embodiment of thepresent invention;

FIG. 3 is a diagram of an LED according to a modification of the secondembodiment of the present invention; and

FIG. 4 is a diagram of an LED according to a third embodiment of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, particularly to FIG. 1, where likereference numerals designate corresponding parts throughout the views, afirst embodiment of the invention will be described. FIG. 1 is aschematic diagram of an LED structure in cross section, and is not toscale but is drawn to illustrate the invention. Reference numeral 10represents a substrate, preferably semi-insulating but n+ and p+substrates may also be used. Disposed on substrate 10 is a p-contactlayer 11. Layer 11 is preferably doped to or near the saturation pointto minimize its resistivity. Layer 11 is also of reduced bandgap toimprove contact resistance.

Disposed on layer 11 is a p-clad/grade layer 12, and disposed on layer12 is an active p-layer 13. The bandgap of layer 13 is selected basedupon the wavelength of light desired to be emitted. Layer 12 has ahigher bandgap than that of layer 13 to obtain confinement of carrierselectrically injected into active layer 13 to improve radiativerecombination efficiency. The grade portion of layer 12 is between theclad portion and contact layer 11 to reduce resistance. Layer 12 mayalso consist of two layers, a clad layer and a grade layer. Also, thebandgap of layer 11 may also be below that of layer 13 and henceadsorptive to photons emitted from the active layer.

Disposed over layer 13 are an n clad/grade layer 14 and an n contactlayer 15. Similar to layer 12, layer 14 has a higher bandgap than thatof layer 13 to obtain confinement of carriers electrically injected toactive layer 13 to improve the radiative recombination efficiency. Layer14 may also be formed of two layers, a clad layer and grade layerbetween the clad layer and the active layer 13. Layer 15 is also ofreduced bandgap to improve contact resistance and may have a bandgapbelow that of layer 13 and hence adsorptive to photons emitted from theactive layer. Layer 15 is preferably made as thin as possible.

Lastly, a metal contact 16 is disposed on layer 15. The contact islocated a desired distance away from the edge of the stack of layers12-15 to provide a desired amount of junction area of the LED. Thisdistance is approximately 15 μm for a typical LED.

In the LED example of FIG. 1, the pn junction is between p active layer13 and n clad/grade layer(s) 14. There is a spreading resistance betweenthe contact on the n contact layer and the pn junction and the p contacton the p contact layer and the pn junction. In a conventional device ona p+ (or n+ substrate) where the contact is on the backside of thesubstrate, there is essentially no spreading resistance since thecontact is directly underneath the junction. This results in a varyingresistance between the two contacts which produces a nonuniform junctionvoltage and nonuniform light emitting characteristics, as discussedabove.

In the present invention, the combined p contact layer 11 and p cladlayer 12 are designed to have the same spreading resistance as thecombined n contact layer 15 and n clad/grade layer 14 so that thejunction voltage will be uniform, or substantially uniform, across thejunction. The spreading resistance may be determined primarily by eitherof the combined p or n layers (layers 11 or 12, 14 or 15). In this casethe other layer could be either thin or lightly doped, or both.Furthermore, the layout of the device is one-dimensional as opposed toradial as is typically done in conventional devices so that thespreading resistances can be more ideally matched.

The sheet resistances of the combined p contact layer 11 and p cladlayer 12 and the combined n contact layer 15 and n clad/grade layer 14may be matched by controlling the composition, doping and/or thicknessesof the layers. In the first embodiment, n-type material on top of thedevice and p-type material underneath the device are utilized since thesheet resistance of n-type material for a given thickness near dopingsaturation is significantly lower than for p-type material due to a muchhigher mobility typically found in the n-type material. This results ina spreading resistivity match for thinner layer on top where photons areemitted than if the n-type material was on bottom. The use of thinnerlayers on the top results in significantly less absorption of emittedphotons since the contact layers are typically of smaller bandgap thanthe active layer. Moreover, this reduced resistivity for the n-typematerial is achieved at a lower doping density which results in lessfree carrier absorption since free carrier absorption is less for n-typematerial than p-type material and the free carrier absorption increaseswith doping density.

Alternatively, the use of n-type material on top allows the spreadingresistance to be reduced for a given tolerable absorption of photonsemitted from the active layer. This lower spreading resistance makes itsignificantly easier to match the spreading resistance and achieve auniform junction voltage since a variation in junction voltage isdetermined primarily by the difference in sheet resistance which causesthe spreading resistance. The current passing through the forward biasedjunction voltage is exponentially dependent on this voltage so a smalldifference in spreading resistance can cause a large variation incurrent across the junction and hence variation in areal brightness fromthe LED. Table 1 below provides estimates of the ratio of currentvariation across a square of material due to a variation in differencein sheet resistance between the upper and lower layers assuming thejunction current is exponentially related to the junction voltage andthe junction voltage is simply determined by a linear IR drop from thecontact. For actual devices the difference values may not be as extreme,based upon actual device design and other considerations includingcurrent flow across the junction.

TABLE 1 Difference in combined layer Sheet Ratio of Current Resistancein Ohms/Square at 10 mA Change Across Junction 100 6 × 10¹⁶ 50 2 × 10⁸ 10 48 5  7 1   1.5

So, for example, the sheet resistance difference between the top contactand the junction and the bottom contact and the junction must bemaintained within or on the order of 1 ohm/square in order to maintainthe current uniformity within a factor of 1.5 across a square ofmaterial, given the above assumptions. In actual devices sheetresistance difference may be maintained within 1-10 ohms/square in orderto maintain currant uniformity sufficient for practical applications.For example, a square of material would be a lateral separation betweentop and bottom contacts of about 15 microns for a contact which was 15microns long (into the page) in FIG. 1.

A more specific example of the first embodiment, an AlGaAs structure,using a semi-insulating substrate with p-type material on the bottom andn-type material on the top, will now be described. Table 2 below givesthe doping concentrations and layer thickness of the AlGaAs structure.It is estimated that a sheet resistance on the order of 15 ohms/squarecan be obtained in the n contact/n clad/n grade layers 14-15 and pcontact layer 11 and p clad layer 12 with a structure described in Table2. Note that this structure is designed to give a nominal emissionwavelength of 650 nm.

TABLE 2 Layer Thickness (nm) Doping (cm-3) Al composition (%) n contact0.3 5 × 10¹⁸ 0.0 n grade 0.1 2 × 10¹⁸ 0.7-0.0 n clad 0.5 2 × 10¹⁸ 0.7 pactive 0.2 2 × 10¹⁷ 0.38-0.39 p clad 0.3 2 × 10¹⁹ 0.7 p grade 0.1 5 ×10¹⁹ 0.0-0.7 p contact 0.5 1 × 10²⁰ 0.0

Although the spreading resistance of the top and bottom layers in theAlGaAs example is likely to be higher than that achieved in prior artLED structures, the sheet resistance of 15 ohms/square is considered tobe quite acceptable given that a ⁻10 percent control of sheet resistanceis considered achievable in production and expected to allow the currentuniformity due to junction voltage variation to be within a factor oftwo. This is a considerable improvement over using a p+ substrate and acombined 15 ohms/square n sheet resistance since this would essentiallyyield a difference in combined sheet resistance of 15 ohms/square and aresulting currently nonuniformity as high as approximately 50 (fromTABLE 1). Furthermore, the AlGaAs structure is only ⁻2 microns in totalheight which simplifies processing and reduces cost when fabricatingarrays of LEDs, versus typical discrete LEDs which may be over 10 μmtotal height.

While the first embodiment is an n over p structure, the presentinvention is not so limited. The invention is equally applicable to a pover n structure. The invention is also applicable to many types ofsemiconductor materials, such as GaAsP, GaInP, AlGaInP, GaInAs, InP andGaN or other combinations of III/V materials.

The structure of the first embodiment can be varied to achieved furtherimproved results. For example, an additional improvement of thestructure is to further thin n contact layer 15 and to thicken andincrease the doping of the n clad region 14 to reduce bandedgeabsorption by layer 15. In the AlGaAs structure, the n contact may bethinned to ⁻0.1 μm or less for high doping concentration.

It is desirable to minimize the absorption in the region above an activelayer between the metal contacts where light is emitted. For thestructure of FIG. 1, this corresponds to layers 14 and 15 (or layer 11for a p over n structure). This can be accomplished in a number of ways.For example, in a second embodiment of the present invention shown inFIG. 2, the n contact layer 15 and the n grade layer 14B have beenremoved in the main portion of light emitting area. Note in FIG. 2 thatseparate clad and grade layers 12A, 12B, 14A and 14B are illustrated.The matching of the spreading resistance is accomplished with the n cladlayer 14A and the p contact layer 11 and p clad/grade 12A and 12B.

In layer 14, only part of the grade composition will be adsorbent to theemitted light. Thus, it is possible to eliminate only the adsorbent partof the grade layer. In a modification of the second embodiment, shown inFIG. 3, n grade layer 14B has been etched to leave that portion 18 thatdoes not absorb, or does not appreciably absorb (to a desired level),emitted light. In the more specific example of TABLE 2, this correspondsto etching that portion of the n AlGaAs grade layer having an Alcomposition comparable or less than the active region, i.e., 0.38-0.39.A desired thickness of the layer may be removed by etching for adetermined amount of time, knowing the etching characteristics andthickness of the grade layer.

Minimizing light adsorption can also be accomplished as shown in FIG. 4,a third embodiment of the device according to the invention. Here, pclad layer includes a ¼ wavelength stack 19. Stack 19 may beimplemented, for example in the device described in TABLE 2, as layersof alternating Al composition such that the composition of all thelayers of the stack is greater than active layer 13, so that there isnegligible adsorption, and that there is sufficient difference in Alcomposition between the alternating layers to achieve reflection oflight generated in the active region and propagating toward thesubstrate. For example, a sufficient difference is in the range of 50-90per cent.

While the fourth embodiment shows etched layers 14B and 15,corresponding to the second embodiment (FIG. 2), the inclusion of layer19 is not so limited. Layer 19 may be used in the other embodiments ofthe invention.

The method of fabricating light emitting devices according to theinvention may be carried out using conventional growth or deposition(MBE, CVD or MOCVD, for example), photolithography, implantation,etching and metallization steps. For example, in first embodiment of themethod according to the invention, the device shown in FIG. 1 may befabricated by growing or depositing layers 11-15 on substrate 10. A maskcan then be formed on layer 15 using photolithography and used to etchlayers 12-15 to expose layer 11 for contact formation. The mask isremoved followed by the forming of contacts 16 and 17. Appropriateannealing steps, or other steps, may be included as needed or as desiredto achieve particular device characteristics.

In a second embodiment of the method according to the invention, used tofabricate the devices of FIGS. 2 and 3, the processing is the same up tothe contact formation. Here, another mask may be formed on layers 11 and15 to facilitate the etching of layers 15 and 14B. Layer 14B may becompletely (FIG. 2) or partially removed (FIG. 3). Contacts 16 and 17may then be formed. Alternatively, contact 16 may be used as a mask,coupled with some type of masking of contact 17 and layer 11 to avoidetching layer 11.

In a third embodiment of the method according to the invention, eitherthe first or second embodiments may be modified to include the growingor depositing of a ¼ wavelength stack between layers 12A and 12B, suchas by growing or depositing alternating layers of different Alcomposition.

In each of the embodiments of the method, the spreading resistancematching is achieved by selecting the growth parameters such as gascomposition, growth times, growth or deposition method, or dopingmethod, to produce a layer of the desired doping concentration andthickness. The layers are typically formed epitaxially to achieveuniform characteristics, such as doping and thickness, to produceuniform spreading resistance values.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein. For example, the invention is not limited to LED devices orfabricating LED devices. The invention is applicable to any lightemitting device where brightness may be limited or affected by spreadingresistance. One such device is a laser.

What is claimed is:
 1. A light emitting device, comprising: a light emitting region; a first contact to said region comprising a first semiconductor material having a first non-negligible spreading resistance; and a second contact to said region, opposing said first contact, comprising a second semiconductor material having a second spreading resistance substantially equal to said first spreading resistance.
 2. A device as recited in claim 1, wherein said light emitting region comprises a pn junction; said first contact comprises at least one layer connected to one of p and n regions of said junction; and said second contact comprises at least one layer connected to the other of said p and n regions of said junction.
 3. A device as recited in claim 2, wherein at least one of said first and said second contacts comprises at least one of a contact, clad and grade layers.
 4. A device as recited in claim 2, wherein said second contact comprises: a clad layer disposed over a light emitting portion of said device; and at least one of a grade layer portion and a contact layer portion disposed over only a desired portion of said grade layer.
 5. A device as recited in claim 4, wherein said grade layer is comprised of a material that substantially does not absorb light emitted from said region.
 6. A device as recited in claim 2, wherein said second contact comprises: a clad layer disposed over said region; a grade layer disposed over said clad layer and over a light emitting portion of said device; and a contact layer portion disposed over only a desired portion of said grade layer.
 7. A device as recited in claim 2, further comprising: a ¼ wavelength layer disposed in one said first and second contacts.
 8. A device as recited in claim 2, further comprising a semi-insulating substrate, wherein: said first contact comprises: a p-type contact layer, a p-type grade layer disposed on said p-type contact layer, and a p-type clad layer disposed on said p-type grade layer; and said second contact layer comprises: an n type clad layer disposed on a p-type active layer, an n type grade layer disposed on said n type clad layer, and an n type contact layer disposed on said n type grade layer.
 9. A device as recited in claim 1, wherein said first spreading resistance is within approximately a range of 1-10 Ω/square of said second spreading resistance.
 10. A device as recited in claim 9, wherein one of said first and second spreading resistances is in the range of approximately 2-50 Ω/square.
 11. A device as recited in claim 1, wherein: said region comprises a p-type active layer; said first contact comprises a p-type contact layer and a p-type grade/clad layer disposed between said p-type contact layer and said active layer; and said second contact comprises a n-type contact layer and an n-type grade/clad layer disposed between said n-type contact layer and said active layer.
 12. A device as recited in claim 11, wherein: said active layer comprises an AlGaAs layer having a doping of approximately 2×10¹⁸/cm³ and an Al composition of approximately 0.38-0.39 percent; said p-type grade/clad layer comprises an AlGaAs layer having a doping of approximately 5×10¹⁹/cm³ and an Al composition of approximately 0-0.7 percent and an AlGaAs layer having a doping of approximately 2×10¹⁹/cm³ and an Al composition of approximately 0.7 percent; said p-type contact layer comprises an GaAs layer having a doping of approximately 1×10²⁰/cm³; said n-type grade/clad layer comprises an AlGaAs layer having a doping of approximately 2×10¹⁸/cm³ and an Al composition of approximately 0-0.7 percent and an AlGaAs layer having a doping of approximately 2×10¹⁸/cm³ and an Al composition of approximately 0.7 percent; and said n-type contact layer comprises an GaAs layer having a doping of approximately 5×10¹⁸/cm³.
 13. A light emitting device, comprising: a light emitting junction; and first and second contacts to said light emitting junction comprising respective first and second semiconductor material having substantially the same nonzero spreading resistance to provide a substantially uniform voltage difference across a predetermined portion of said junction.
 14. A device as recited in claim 11, wherein one of said contacts has a spreading resistance within approximately a range of 1-10 Ω/square of that of the other contact.
 15. A device as recited in claim 12, wherein one of said contacts has a spreading resistance in the range of approximately 2-50 Ω/square.
 16. A method of fabricating a light emitting device, comprising: forming a light emitting region; forming a first contact to said light emitting region comprising a first semiconductor material having a first non-negligible spreading resistance; and forming a second contact, opposing said first contact, to said light emitting region comprising a second semiconductor material having a second spreading resistance approximately equal to said first spreading resistance.
 17. A method as recited in claim 16, wherein: forming said region comprises forming an active layer; forming said first contact comprises forming a first grade/clad layer connected to said active layer and forming a first contact layer on said first grade/clad layer; and forming said second contact comprises forming a second grade/clad layer connected to said active layer and forming a second contact layer on said second grade/clad layer.
 18. A method as recited in claim 17, comprising: forming said first contact layer and said first grade/clad layer to have said first spreading resistance; and forming said second grade/clad layer and said second contact layer to have said second spreading resistance.
 19. A method as recited in claim 17, comprising: forming said first grade/clad layer over said active layer; forming said first contact layer over said first grade/clad layer; removing a portion of said first contact layer in a light emitting area of said device; and removing at least a portion of said first grade/clad layer in said area.
 20. A method as recited in claim 19, comprising: forming said second contact layer and said second grade/clad layer to have said second spreading resistance; and forming a remaining portion of first grade/clad layer after said step of removing at least a portion and a remaining portion of said first contact layer to have said first spreading resistance.
 21. A method as recited in claim 19, wherein said removing at least a portion step removes substantially that portion of said first grade/clad layer adsorbent to light emitted by said device.
 22. A method as recited in claim 19, comprising: forming said active layer and said first grade/clad layer using compound semiconductor materials; and removing substantially said portion of said first grade/clad layer having a composition absorbent to light emitted by said device.
 23. A method as recited in claim 17, comprising: forming said active layer using a compound semiconductor material having a first bandgap; and forming said first and second grade/clad layers using a compound semiconductor material having a second bandgap greater than said first bandgap.
 24. A method as recited in claim 17, wherein forming said first and second contact comprises controlling at least one of doping, composition and thickness of at least one layer in one of said first and second contacts to make said first and second spreading resistances substantially equal.
 25. A method as recited in claim 16, wherein forming said first contact includes forming a ¼ wavelength stack.
 26. A method as recited in claim 16, comprising: forming said first and second spreading resistances to be approximately in the range of 2-50 Ω/square.
 27. A method as recited in claim 16, comprising: matching said first and second spreading resistances within a range of approximately 1-10 Ω/square.
 28. A method of fabricating a light emitting device, comprising: forming a light emitting junction; and forming contacts to said junction each comprising a semiconductor material such that a substantially uniform voltage is applied across said junction by substantially matching spreading resistances of said contacts.
 29. A method as recited in claim 28, comprising: forming spreading resistances to be in a range of approximately 2-50 Ω/square.
 30. A method as recited in claim 28, comprising: matching said spreading resistances within a range of approximately 1-10 Ω/square.
 31. A method as recited in claim 28, wherein: each of said contacts comprises at least one layer of material; and said matching comprises controlling at least one of doping, composition and thickness of a material of a layer in at least one of said contacts.
 32. A method as recited in claim 31, wherein: said matching comprises controlling at least one of doping, composition and thickness of a material of a layer in each of said contacts.
 33. A light emitting device, comprising: a light emitting region; a first contact to said region having a first non-negligible spreading resistance; and a second contact to said region, opposing said first contact, having a second spreading resistance substantially equal to said first spreading resistance, wherein said first spreading resistance is within approximately a range of 1-10 Ω/square of said second spreading resistance.
 34. A light emitting device, comprising: a light emitting junction; and first and second contacts to said light emitting junction having substantially the same nonzero spreading resistance to provide a substantially uniform voltage difference across a predetermined portion of said junction, wherein one of said contacts has a spreading resistance within approximately a range of 1-10 Ω/square of that of the other contact.
 35. A method of fabricating a light emitting device, comprising: forming a light emitting region; forming a first contact to said light emitting region having a first non-negligible spreading resistance; forming a second contact, opposing said first contact, to said light emitting region having a second spreading resistance approximately equal to said first spreading resistance; and matching said first and second spreading resistances within a range of approximately 1-10 Ω/square.
 36. A method of fabricating a light emitting device, comprising: forming a light emitting junction; forming contacts to said junction such that a substantially uniform voltage is applied across said junction by substantially matching spreading resistances of said contacts; and matching said spreading resistances within a range of approximately 1-10 Ω/square. 